drug absorption and methods of studying drug absorption

1. By
P. SIVA KRISHNA
M. PHARMACY
GIET School of Pharmacy
Andhra University

2. FIRST-PASS EFFECTS
The topics under this phenomenon are:
 Theory
 Systems that effect pre-systemic metabolism
of a drug
 Estimation of bio-availability
 Prevention
 Beneficial effects

3. THEORY
 The first-pass effect is a phenomenon of drug metabolism in which the
concentration of a drug is greatly reduced before it reaches the systemic
circulation.
eg: Imipramine, Morphine , Propranolol, Diazepam, Cemitidine etc.
 The indication of first-pass effect is the diminished drug concentration
or complete absence of drug in the plasma after oral administration.
 The first-pass effect may leads to inactivation or activation of a drug.

4. SYSTEMS AFFECTING PRE-SYSTEMIC
METABOLISM
The systems included are:
 Digestive enzymes
 Bacterial enzymes
 Gut wall enzymes/mucosal enzymes
 Hepatic enzymes

5. DIFFERENT SITES
OF PRE-SYSTEMIC
METABOLISM

6. DIGESTIVE
ENZYMES
• These are present in gut fluids
intestinal and pancreatic secretions
• These include hydrolases which
hydrolyse ester drugs like
chloramphenicol palmitate to active
chloramphenicol
• And peptidases which split amide
linkages and inactive protein or
polypeptide drugs. In order to avoid
this peptide drugs are delivered to
colon which lack peptidases

7. BACTERIAL
ENZYMES
• Microflora are present scantily in
stomach and small intestine and are
rich in colon hence most orally
administered drugs are unaffected by
these enzymes
• Sulfasalazine is converted to
sulfapyridine and 5-aminosalicylate by
the microbial enzymes of colon
• Intestinal microflora hydrolyse
conjugates of drugs that are actively
secreted via bile and the free drugs are
reabsorbed into blood eg: glucuronides
of digoxin and oral contraceptives

8. GUT WALL
ENZYMES
• They are present in stomach, small
intestine and colon
• Alcohol dehydrogenase enzyme
which is present in stomach mucosa
inactivates ethanol
• Sulphation of ethanol oestradiol and
isoprenaline by intestinal enzymes
• Colonic mucosa contains both
phase-l and phase-ll enzymes but
only the enzymes of proximal small
intestine are most active

9. HEPATIC
ENZYMES
• The liver contains both phase-l and
phase-ll enzymes
• This is the major site of metabolism
of most orally given drugs
• Highly metabolized drugs in this are
Isoprenaline, Nitroglycerin,
Diltiazem, Nefidepine, Lidocaine,
Morphine etc..

10. ESTIMATION OF BIOAVAILABILITY
Absolute bioavailability:
• For an orally administered drug that is chemically stable in
the gastrointestinal tract and is 100% systemically absorbed
(F = 1), the area under the plasma drug concentration
curve, AUC∞
0, oral, should be the same when the same drug
dose is given intravenously, AUC∞
0, IV.
• Therefore, the absolute bioavailability (F) may reveal evidence
of drug being removed by the liver due to first-pass effects
as follows
 
  oraliv
ivoral
DAUC
DAUC
F 

11. Liver extraction ratio:
• There are many other reasons for a drug to have a reduced
F value, the extent of first-pass effects is not very precisely
measured from the F value
• The liver extraction ratio (ER) provides a direct
measurement of drug removal from the liver after oral
administration of a drug.
ER=(Ca-Cv)/Cv
C a is the drug concentration in the blood entering the liver
C v is the drug concentration leaving the liver

12. Relationship b/w F and ER
• Sampling of drug from the hepatic portal vein and artery is
difficult so the following relationship between bioavailability and
liver extraction enables a rough estimate of the extent of liver
extraction
F=1-ER-F’’
F” is the fraction of drug removed by nonhepatic process
F is the fraction of bioavailable drug
• If F” is considered negligible then the equation is as follows
F=1-ER then
ER=1-F

13. PREVENTION:
• We can avoid first pass effect by choosing alternative routes
of administration like suppositories, I.V, I.M, inhalational
aerosol and sublingual
• Because they allow drug to be absorbed directly into
systemic circulation
• Prevention is taken only for drugs that are destroyed by the
first-pass effect

14. BENIFICIAL EFFECTS:
• Prodrugs like codeine(methylmorphine) are converted from
inactive form to pharmacologically active form morphine
• Aspirin a prodrug which is inactive is converted to active
salicylic acid by esterase's in liver
• Sulphasalzine an inactive form converted to sulphapyridine
and active 5-aminosalicylate

15. METHODS FOR STUDYING DRUG
ABSORPTION
Classification :
 In silico methods
 Physicochemical methods
 In vitro methods
 In situ methods

16. IN SILICO METHODS
• In silico methods predict intestinal permeability on the basis
of drug characteristics or descriptors, such as lipophilicity,
H-bonding capacity, molecular size, polar surface area, and
quantum properties.
• One of the most famous is the Rule of five by Lipinski et al
• This approach predicts low permeability or solubility when
the following cut-offs are exceeded:

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1. There are more than five H-bond donors (expressed as the sum of OHs
and NHs).
2. The molecular weight MW is over 500.
3. The log P is over 5 (P is the drug 1-octanol/water partition coefficient).
4. There are more than 10 H-bond acceptors (expressed as the sum of Ns
and Os).
5. Compound classes that are substrates for biological transporters are
exceptions to the rule.
• The Rule of five has the advantages of being simple, easy to interpret,
and fast to compute even if, as other similar approaches [80], it does
not take into account the interactions between drug properties

18. PHYSICOCHEMICAL METHODS
• Dressman et al. propose to incorporate various basic drug
physicochemical properties into a unique parameter, the
absorption potential (AP), defined by
AP=log(PFun/D0), D0=X0/CsVL
• P is the drug 1-octanol/water partition coefficient, Fun is the
fraction of drug not ionized at pH 6.5, Cs is the aqueous
solubility of not ionized species at 37 C, VL is the volume of
water taken with the dose (it is usually set = 250 ml), X0 is
the drug dose, and D0 is dose number.

19. IN VITRO METHODS
• In vitro techniques when compared to vivo methods they do
not account for the effect of physiological factors such as
gastric emptying rate, gastrointestinal transit rate,
gastrointestinal pH, etc
In vitro strategies :
1.Animal tissue methods :
a)Everted gut technique
b)Intestinal sheets
c)Isolated membrane vesicles
2.cell based methods : Caco-2 cell method

20. EVERTED GUT
TECHNIQUE

21. EVERTED GUT TECHNIQUE
• It is aimed to determine intestinal wall permeability on the
basis of drug transport across the membrane from the donor
drug solution to the receiver environment
• This technique implies fixing the intestinal tract on the left
side of a ‘‘U’’ glass capillary connected to a cylindrical glass
vessel. In this disposition, intestinal mucosa faces the donor
environment while the serosal side faces the receiver
environment.
• This model is ideal for studying the absorption mechanism of
drugs since both the passive and active transport can be
studied.

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• The whole preparation is maintained at 370
C and shaken
mildly.
• At predetermined time intervals the sac is removed and the
concentration of drug in the serosal liquid is determined.
• The advantage of this configuration consists in a wide donor
volume and a small receiver volume so that drugs
accumulate faster.
• The drawbacks of this technique are the lack of active blood
and nerve supply that can lead to a rapid loss of viability
and tissue damage due to intestine everting.

23. INTESTINAL SHEETS
• Involves the isolation of the intestinal tissues, cutting it into
strips of appropriate size and clamping it on a suitable
device so that the donor and receiver environments are
separated by a flat intestinal membrane.
• Tissue permeability is evaluated resorting to drug
concentration increase in the receiver environment (that is
in contact with the intestine serosal side).
• Lack of blood and nerve supply, rapid loss of tissues viability,
changes in morphology and functionality represent main
drawbacks of this approach.

24. ISOLATED MEMBRANE VESICLES
• Membrane vesicles can be prepared from either intestinal
scrapings or isolated enterocytes
• They allow studying drug and nutrient transport at the
cellular level (brush border as well as basolateral side), they
are ideal for mechanistic absorption studies and for the
isolation and identification of transporter proteins
• One of the most important advantages of vesicles over the
everted gut and intestinal sheets approaches is the very
small amount of drug required.

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• In addition, the possibility of vesicles cryopreservation
permits storing them for a long time.
• Conversely, it is practically impossible to isolate pure brush
border membrane vesicles (or basolateral vesicles) without
the contamination with the other type of vesicles.
• Vesicles isolation process often implies transporter proteins
and enzymes getting damaging.

26. CELL BASED METHODS
• some of the most commonly used cell models we can mention
Caco-2 (human/colon; epithelial), HT-29 (human/colon;
epithelial), T-84 (human/colon; epithelial), MDCK (canine/kidney;
epithelial), and LLC-PK1 (porcine/kidney; epithelial).
• Caco-2, is human colon adenocarcinoma undergoing spontaneous
enterocytic differentiation leading to a monolayer (on a
semipermeable porous filter) where cell polarity and tight
junctions are well established.
• In addition, as they do not produce mucus, permeability
evaluation is altered by the absence of the diffusive resistance
offered by mucus
• Finally, it is worth mentioning that Caco-2 cell model accounts
only for passive drug transport

27. IN SITU METHODS
• These methods simulates the in vivo conditions for drug absorption
and are based on perfusion of a segment of GIT by drug solution
and determination of amount of drug diffused through it.
• The most important advantage of in situ methods compared to the
in vitro techniques consists in intact blood and nerve supply.
• Accordingly, this methodology is highly accurate for predicting the
permeability of passively transported compounds, while the use of a
scaling factor is recommended for predicting permeability of
carrier-mediated compounds
• Classification :
1)Doluisoi method
2)Single pass perfusion technique

28. DOLUISIO
METHOD

29. DOLUISIO METHOD
• In this method, the upper and lower parts of the small
intestine of anaesthetised and dissected rat are connected by
means of tubing to syringes of capacity 10 – 30 ml.
• After washing the intestinal segment with normal saline, the
syringe is filled with a solution of radiolabelled drug and a
non-absorbable marker which is used as an indicator of
water flux during perfusion.
• Part of the content of the syringe containing drug is
delivered to the intestinal segment which is then collected in
the second syringe and analysed for drug.

30. SINGLE PASS
PERFUSION
TECHNIQUE

31. SINGLE PASS PERFUSION TECHNIQUE
• It is generally considered superior to the Doluisio method
giving better control of the hydrodynamics and increased
surface area
• The drug solution is perfused continuously (via an infusion
pump) down a set length of intestine through the
duodenalend cannula and perfusate collected from the ileal-
end cannula, at flow rates of between 0.1 and 0.3 ml/min.
• The samples collected at outflow are assayed for drug
content.

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